The armpit of something
Definition 1 of 1
Pro Tip 1/3Negative Connotation
The phrase indicates strong negativity towards a place and can be offensive if used carelessly. 
Referring to my hometown as 'the armpit of the state' really hurt my feelings.
Pro Tip 2/3Informal Slang
It's casual speech and might be inappropriate in formal writing or professional settings. 
His business letter was poorly received because he called the location 'the armpit of the coast'.
Pro Tip 3/3Subjective View
It reflects a personal opinion, so others might disagree with using this phrase for a particular place. 
I think you're wrong in calling this place 'the armpit of the region'; it has its charm!